1. Field of the Invention
The invention relates to preform cutting elements for rotary drag-type drill bits, of the kind comprising a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to the front surface of a substrate which is less hard than the superhard material.
2. Description of Related Art
Such preform cutting elements usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a pocket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a pocket in the bit body, without being first brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be molded using a powder metallurgy process.
In preform cutting elements of the above type the interface between the superhard table and the substrate may be flat and planar. However, the bond between the superhard facing table and the substrate may be improved by providing a configured non-planar interface between the rear face of the facing table and the front face of the substrate, so as to provide a degree of mechanical interlocking between the facing table and substrate.
In such preform cutting elements it is the usual practice for the facing table to extend over the whole area of the front face of the substrate so that the periphery of the facing table is exposed at the periphery of the preform element.
Such preform cutting elements are subjected to high temperatures and heavy loads when the drill bit on which they are mounted is in use down a borehole. It is found that as a result of such conditions spalling and delamination of the superhard facing table can occur, that is to say the separation and loss of the diamond or other superhard material over part or all of the cutting surface of the table.
It is believed that impact loads on the cutting edge of the facing table can initiate such spalling or delamination and such impacts can also cause cracking of the superhard facing table, which tends to be comparatively brittle.
Another disadvantage of existing designs is that when the cutting element is brazed into a pocket in the bit body, the braze alloy cannot wet or bond to the portion of the facing table which is exposed at the periphery of the cutting element. This not only reduces the effective braze area but also means that the bit body provides no effective support for the facing table at its periphery.
The present invention sets out to provide a novel and improved form of cutting element where these disadvantages may be reduced or overcome.